VCSEL devices are laser diode devices used in a variety of applications to generate optical signals. For example, in optical communications networks, VCSEL devices are often used to generate optical information signals that are transmitted over optical fibers of the network. The speed at which a VCSEL device can be driven, or modulated, ultimately is limited by the onset of relaxation oscillation inherent to the operation of the VCSEL device. In such devices, relaxation oscillation is a manifestation of the energy exchanged between the total photon and carrier populations when the laser is disturbed from a steady state condition. This energy exchange results in a damped optical output power oscillation at the relaxation oscillation frequency. The relaxation oscillation frequency is a function of the square root of the laser bias current. In general, the relaxation oscillation frequency, fR, for a given bias current, is relatable to the maximum modulation frequency bandwidth at which a laser diode can be driven, defined by the figure of merit f3dB, by the expression:f3dB˜1.55*fR.Thus, the 3-decibel (dB) modulation bandwidth of the laser diode is limited to a value of about 1.55 times the relaxation oscillation frequency.
Although the intrinsic speed, or bandwidth, of materials currently used in VCSEL devices can be as high as 40 gibabits per second (Gbps), many technological factors have thus far made it difficult to overcome the limitations imposed by relaxation oscillation and extend VCSEL speed beyond about 10 Gbps. One technique that has been employed in laser diodes to overcome limitations imposed by relaxation oscillation involves using optical feedback to extend bandwidth.
For example, one proposed optical feedback structure for a VCSEL device employs a curved mirror to produce optical feedback that results in enhancement of the bandwidth of the VCSEL device. The proposed implementation, however, has several drawbacks. First, the VCSEL device substrate makes up part of the optical feedback structure and has an effect on the optical feedback that is used to extend the bandwidth. Consequently, the operating wavelength of the VCSEL device is limited to wavelengths that are longer than the absorption band edge wavelength of the substrate material. For example, if a gallium arsenide (GaAs) substrate material is used, the device will need to operate at a wavelength that is longer than 870 nanometers (nm) (bandgap energy 1.424 eV for GaAs). This wavelength is longer than the 850 nm wavelength most commonly used in optical communications networks. Therefore, the proposed VCSEL device would not be suitable for use in most optical communications networks.
Second, the curved mirror must be formed to provide adequate optical feedback to a laser cavity located at a distance from the mirror that is more than ten times the size of the laser aperture. Fabricating such a mirror with the necessary precision would be very difficult, if not impossible, and would introduce additional complexity into the VCSEL device fabrication process.
Third, the proposed implementation for the VCSEL device requires controlling the thickness of the die used in the process with a precision of less than one wavelength in order to provide the necessary phase control of the optical feedback. For example, for a 900 nm wavelength, the die thickness would need to be controlled to a precision of less than 30 nm. Such precision is very difficult, if not impossible, to achieve with current state-of-the-art lapping and polishing methods used to thin semiconductor substrates.
The use of optical feedback to extend bandwidth has also been employed in edge emitting lasers. Such devices generally rely on mode coupling between the main laser mode and a side resonant mode, resulting in the formation of an additional resonance peak at a higher frequency in the radio frequency (RF) spectrum. Coupling between the modes, sometimes referred to as “push-pull”, operates to generate additional resonance conditions in the higher frequency regime, thereby enhancing its overall modulation bandwidth.
The coupled cavity designs employed in edge emitting laser diodes have several disadvantages. The designs are typically realized by creating a coupled feedback cavity comprising either a long passive distributed Bragg reflector (DBR) structure or an external mirror configuration. The external cavities tend to be expensive, extremely sensitive to system setup stability and generally suitable only for laboratory use. The DBR comprises a grating structure that is manufactured using either electron beam (e-beam) lithography or holographic writing techniques, which are expensive to perform and have relatively low throughput. In addition, edge emitting lasers can only be tested after facet cleaving, which results in higher testing costs and lower yield.
Other edge emitting laser diode devices that use optical feedback to extend bandwidth do not rely on mode coupling, but instead rely on shifting of the Fabrey-Perot (FP) resonant mode wavelength of the device relative to the reflectivity maximum wavelength of the device. This shifting of the FP mode resonant wavelength (i.e., the lasing wavelength) is often referred to as “detuning”. In these devices, the amplitude and phase of the optical feedback are controlled to cause the lasing wavelength to be detuned by a positive amount relative to the maximum reflectivity wavelength. This positive detuning results in the maximum modulation bandwidth of the device being extended.
One of the disadvantages of edge emitting laser diode devices that use detuning to enhance bandwidth is that the DBR is implemented as a relatively long grating structure that is relatively expensive to produce. Growing the grating structure requires the use of multiple epitaxial growth steps, which increases process complexity and costs. In addition, as stated above, edge emitting lasers can only be tested after facet cleaving, which results in higher testing costs and lower yield.
VCSEL devices are generally capable of being manufactured with higher yield and lower manufacturing costs than edge emitting lasers, due in part to the fact that they can be tested directly at the wafer level. Coupled active cavity resonance designs have also been proposed in VCSEL devices for extending bandwidth. Such designs, however, do not rely on principles of optical feedback, but rather, on interactions between the photon population and independent electron populations residing in multiple active multi-quantum well (MQW) cavities in the device to increase the relaxation oscillation frequency, fR, which indirectly results in an increase in f3dB in accordance with the above equation. One of the disadvantages of this type of design is that use of the multiple active cavities increases the complexity of the biasing circuitry, which increases manufacturing costs. Another disadvantage is that bandwidth enhancement is predictable only for a narrow set of parameters and is difficult to achieve experimentally.